This invention relates to a cast-iron product having high damping capacity for use in a brake disc, a brake drum and so forth, and a method of producing such a cast-iron product.
In recent years, attempts have been made to reduce an undesirable brake noise occurring upon the braking of a vehicle. This is an important technical problem, and solving of this problem would lead to an improvement in, and riding comfort of drivers and passengers.
One approach for reducing the brake noise is to redesign the whole construction of the brake system so that the rigidity of materials is so changed as to have the resonant frequency shifted. This method might contribute to a reduction in brake noise in a relatively low range of vibration frequency. However, in this approach, it is difficult to obtain a sufficient reduction in brake noise in a range of high frequency, i.e., above 4 Hz.
Lately, brake discs or brake drums tend to be made of cast iron, which has a relatively excellent vibration damping capacity. It is a known fact that the damping capacity of cast iron is generally enhanced as the carbon equivalent (CE) represented by the following formula is increased; ##EQU1##
Cast iron which is made by the application of this fact has been already proposed by Japanese Patent Publication No. 56-42663, the Patent of which was issued to Mitsubishi Heavy Industry Co., Ltd. On the other hand, it is also a known fact that a higher carbon equivalent in cast iron leads to lower strength and hardness of the cast iron.
In the Patent issued to Mitsubishi Heavy Industry Co., Ltd., the cast iron contains alloying elements such as Chromium (Cr) and Zirconium (Zr), which are added to the cast iron and are intended to improve the strength and hardness thereof. However, under circumstances in which increments of engine power output and higher quality of parts or assemblies in the whole vehicle structure are desired, it has been found that the above cast iron is not sufficient to provide brake parts having a performance ability or quality equal to or greater than the other parts and assemblies of the vehicle.
Another alloying element such as Antimony (Sb) might be added to the cast iron. Such a cast iron, however, would generate undesirable gases such as SbO.sub.3 and, therefore, the same could not be practically used at the present stage.
A brake assembly in use for a braking system of a vehicle comprises, in general, a sliding element referred to as a brake disc or a brake drum, and a friction element, such as a brake pad or a brake shoe which is brought into sliding contact with the sliding element. In order to reduce the brake noise, it is required not only to pay attention to enhancement of the damping capacity of the sliding element, but also to be careful about what kind of combination of the sliding element and the friction element is adequate.
In a case where asbestos is used for the friction element, the friction element has excellent braking characteristics, as well as characteristics desired for noise elimination regardless of the materials of the sliding element. For this reason, friction elements made of asbestos have been used for many years. Asbestos, however, has been recently found to be a material which might negatively affect human health, when in the form of dust. To prevent such environmental pollution, friction elements are required to be made of non-asbestos materials. In response to this requirement, there has been a tendency for friction elements to be made of non-asbestos materials, such as a ceramic material containing a predetermined amount of steel fibers.
In a case where such a friction element is made of any semi-metallic material containing steel fibres, the coefficient of friction between the friction element and the rubbing surface of the brake disc or drum has been found to be greater than the case where the friction element is made of asbestos. In addition, the coefficient of friction has been found to increase as the rotational speed of a wheel decreases. As can be inferred from these facts, such a semi-metallic element results in the frequent occurrence of brake noise.
Taking such facts into consideration, it has been deduced that, in a case where any semi-metallic material is to be used as the friction element, it is better for the brake disc or drum to be made of any material possessing a higher vibration damping capacity.